Agenda Item: 620-2007 Rev 8Title: API 620 ‘Low-Stress /Low Temperature’ Design Allowance Inclusion /Extension

Date: 2013 (~ original date)

Contact: Name : Rick Simmons (originally submitted by David Nadel of Chevron)Company : BechtelPhone : (713) 235-3963 (mobile 630-728-1205)E-mail : jsimmons@bechtel.com

Purpose: Modify API 620, including Annex R and Q to allow the ‘low-stress /low-temperature design basis’ as currently allowed by ASME Section VIII UCS 65 through 68 Div2 revision 2017 (hereinafter referred to as ‘BPVC VIII-2-2017’) Sections 3.11.2.1, 3.11.2.3, 3.11.2.5, 3.11.2.6, 3.11.2.9 and 3.11.2.10, with some limitations specific to API 620 type of components, such as disallowing its use for lap welded components. Reasons for excluded paragraphs of section 3.11.2 are:3.11.2.2 - This section is for thickness >4” and high yield strength > 450 MPa;3.11.2.4 - This section covers flanges and non-API steels;3.11.2.7 - This section covers operating a pressure vessel at below the name plate designated MDMT;3.11.2.8 - This section covers the Code user performing his own fracture mechanics work /approach.

Possible uses for this API 602 Standard inclusion /extension:Auto cooling as suggested by BPVC VIII-2-2017 3.11.2.6 b) is one use. Another use can be for cold tanks with upper cylindrical rings which are nominally stressed.

Source: Originally submitted by David Nadel of Chevron, in the RTTG. This has been discussed for several years in various stages of completion. Previous drafts have been circulated to: L. Hiner, D. Comire, D Nadel, N Stokes, D Miller and R Challa, with comments addressed. David Nadel handed this off to Rick Simmons in 2017. It has been generally agreed to reference ASME rather than include paragraphs and figures into API.

Revision: 6 (date of this revision 01May2018)7 Change from using ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1 to using BPVC VIII-2-2017 Sections 3.11.2.1, 3.11.2.3, 3.11.2.5, 3.11.2.6, 3.11.2.9 and 3.11.2.10, since two negative voters have advised that BPVC Section VIII UCS 65-68 can be very unconservative in some non-PWHT cases, but ASME BPVC VIII-2-2017 has been updated to reflect more modern fracture mechanic results, and for the non-PWHT cases is typically quite a bit more conservative than Division 1.8 McDermott’s Oct2018 Comments Addressed

Impact: The business impact of this item may be significant.

Rationale: Revise API 620 to allow using the low-stress /low-temperature basis /philosophy of ASME VIII, which has been an allowance within ASME for many years.

This Agenda Item is now being co-sponsored by Rick Simmons (and assisted by Larry Hiner and Neville Stokes), having previously been promoted /sponsored by David Nadel of Chevron. More recently, Mike Osterfoss and Don Comire have provided timely and valuable assistance.

It is noted that the current version of API 620 currently has very limited allowance for ‘low-stress’ design, but this Agenda item allows a much broader of use of low-stress /low-temperature design, as has been allowed by ASME VIII Div 1 for many years. It is noted that ASME BPVC VIII-2, unlike API 620, follows the format of showing SI units first, with UCS units following in parenthesis.

This Proposal follows the basis of ASME BPVC VIII-2-2017 Sections 3.11.2.1, 3.11.2.3, 3.11.2.5, 3.11.2.6, 3.11.2.9 and 3.11.2.10 ASME VIII Div 1, UCS-65 through UCS-68 which covers carbon steel materials for low temperature operation down to as low as -155

ºF (-104 ºC), with UGS-66 (b) (2), UGS-66 (b) (3), and UGS-66 (b) (i) specific to allowing lower minimum design metal temperatures with or without impact testing when the actual tensile stress is less than the allowable tensile stress, as well as using material below its actual impact tested temperature, subject to the other provisions of UCS-65 through UCS-68. with 3.11.2.3 being for a colder MDMT with thickness and material specification consideration; 3.11.2.5 for a colder MDMT based on stress; and 3.11.2.6 for a colder MDMT for impact tested material with lower stress.

The user will be required to follow all of the relevant requirements of ASME BPVC VIII-2-2017 when utilizing the cited ASME MDMT provisions, including but not limited to the lower allowable stress, material specification requirements, impact exemption curves, and PWHT requirements.

Modifications will need to be made to API 620 main body, Annex R for low temperature, and Annex Q for colder than ‘low temperature’ since outer tank components are often partly or wholly made of carbon steel and for warmer products some of the components may be designed utilizing this allowance; thus, the engineer may wish to utilize the low-stress /low-temperature allowances for some of those components. For low-stress /low-temperature design of R and Q components, it is the intent for the user to refer back to the main body of 620 rather than putting in the same design requirements/ criteria into API 620 at three separate locations (base API 620, Annex R, and Annex Q).

The allowance for ‘low-stress /low temperature’ design provisions are intended to be available to the design engineer for any load case /component (excluding certain lap welded components).

It is intended to ‘shoe horn’ changes into API 620 only as needed to allow full use of ‘low-stress /low-temperature’ design methodology, keeping the number of changes to an absolute minimum. It is intended to avoid re-writing or re-organizing any current paragraphs of API 620, but there are several paragraphs which will need adjustment so this all hangs together. However, having said that, a few associated paragraphs will need some clean-up work due to referencing, and also to update wording to such as ‘warmer’ or ‘colder’ instead of less or more or higher or lower (referring to temperature).

During the most update of this proposal mid-June 2017, L Hiner pointed out that during any modification to the scope limitations on Design Metal Temperature, we need to avoid limiting or negating the allowance in the second sentence of 4.2.5.1 “Any material listed in Table 4-1 may be used for any thickness and temperature provided the material meets impact test requirements as specified in 4.2.5 and Table 4-3 and welding procedure requirements specified in 6.7.”. So, the modifications are specifically intended to leave this allowance intact; thus, the allowance for low one day mean temperature modified to cover most of the planet, with the modifications to Table 4-1 and adjustments to 4.2.1.2 relative to how the Design Metal Temperature is specified.

During the recent Spring API Refinery and Equipment Standards Meetings in Seattle, this Agenda Item was discussed in the Committee settings (RTTG and the Sub-Group Fab), and it was agreed to bring this Agenda Item to letter ballot. Adjustments to some of the wording was incorporated as a result of the discussions.

Proposal: Revised Sections Only Shown, revisions noted in color:

API 620 Main Body:

1.2.3 The basic rules in this standard provide for installation in areas where the lowest recorded 1-day meanatmospheric temperature is –70 °F or warmer 1, and the designer will additionally have the availability to use the provisions of section 4.2.2 Low-Stress/ Low-Temperature Design down to as cold as -155 ºF. Annex S covers stainless steel low-pressure storage tanks in ambient temperature service in all areas, without limit on low temperatures. Annex R covers low-pressure storage tanks for refrigerated products at temperatures from +40 °F to –60 °F. Annex Q covers low-pressure storage tanks for liquefied gases at temperatures not colder than –325 °F.

1 The basic rules in this standard were originally written for use within the contiguous 48 United States of America, where the coldest recorded low 1-day mean atmospheric temperature is approximately -50 ºF, which would result in a Design Metal Temperature of -35 ºF ( -50 ºF + 15 ºF = -35 ºF) when using the +15 ºF addition per 4.2.1.2 b). Thus, past issues of API 620 Table 4-1 only provided for design metal temperatures as cold as -35 ºF, but allowances for colder temperatures are now incorporated with this update.

4.2.1.2

a) Except as allowed by low-stress and /or low-temperature provisions of 4.2.2, plates shall conform to one or more of the specifications listed in Table 4-1 as being acceptable for use at that temperature (Refer to 4.2.5 for impact testing requirements).

b) For steel outer tanks of refrigerated tank systems to API 625, design metal temperature shall be in accordance with API 625, section 6

c) Except for steel outer tanks of refrigerated tank systems to API 625, the design metal temperature shall be determined by one of the following:i) Specified by the Purchaserii) For materials in contact with nonrefrigerated fluids, the design metal temperature shall be assumed to be 15 °F

above the lowest one-day mean ambient temperature for the locality involved, as determined from Figure 4-1 or by the most reliable historical meteorological data available.

iii) For materials where special situations are in place (such as an externally insulated tank, heated product tank, or for materials not in contact with the product), the design metal temperature shall be determined by analysis /computations or by actual temperature data on similar tanks in similar service.

Table 4-1 Extend the table as follows:

Colder than -35 ºF but warmer than or equal to -55 ºF

Thicknesses as designated /allowed above, or as allowed by the references in 4.2.2

At the DMT, any material specification and grade listed above when both of the following are satisfied: a) the impact requirements in 4.2.5 and Table 4-3 with 5 ft-lbs added to the minimums are satisfied, and b) the weld procedure requirements in 6.7 are satisfied.

Alternatively, the low-stress /low-temperature allowances per 4.2.2 may be utilized.

4.2.2 Low-stress/ Low Temperature Design Provisions

Either of the following a) or b) may be utilized:

a) The following design criteria, relative to the use of Table 4-1, apply when the actual stress under design conditionsdoes not exceed one-third of the allowable tensile stress.

1) Consideration of the design metal temperature is not required in selecting material from Table 4-1 for tankcomponents that are not in contact with the liquid or vapor being stored and are not designed to contain thecontents of an inner tank (see Q.2.4 and R.2.3).

2) The design metal temperature may be increased by 30 °F in selecting material from Table 4-1 for tankcomponents conforming to any of the following:

i. components exposed to the vapor from the liquid or vapor being stored and are not designed to contain thecontents of the inner tank;ii. components located within, but not welded directly to, a primary liquid container, a secondary liquid container, ora warm product vapor container (see R.2.4).

3) Excluding bottom plates welded to the cylindrical sidewall of flat-bottom tanks, the plates of a non-refrigerated flat-bottom tank, counterbalanced in accordance with 5.11.2, may be constructed of any material selected from Table4-1.

b) Except for lap welded bottoms, lap welded suspended decks, lap welded roofs, and lap welded liner plates for concrete walls, the provisions of ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1 ‘Low Temperature Operation’ Paragraphs UCS-65 through UCS-68 may be utilized. In this case, the requirements of ASME specific to satisfying the provisions of UCS-65 through UCS-68 shall be utilized 2 issue year 2017 (hereinafter referred to as BPVC VIII-2-2017) Sections 3.11.2.1 (general); 3.11.2.3 (Exemption From Impact Testing based on the MDMT, Thickness, and Material Specification); 3.11.2.5 (Exemption From Impact Testing Based on Design Stress Values); 3.11.2.6 (Adjusting the MDMT for Impact Tested Materials); 3.11.2.9 (PWHT); and 3.11.2.10 (weld procedure impact testing) together with any referenced paragraphs or sections may be utilized for material design of components, including but not limited to the allowable stresses, material specifications, locations requiring post-weld heat treat, IMPACT TEST TEMPERATURES, IMPACT TEST EXEMPTION TEMPERATURES, and impact energy value requirements, with the following clarifications:- NDE methods and acceptance criteria may follow Section 7 of this Standard.- The utilization of BVPC VIII-2-2017 may be by individual components, but for connected components which are materially different, such as annular plate attached to first shell ring, the most stringent criteria of the connected components applies to both parts.- Allowable stresses from API 620 still applies to component design, even when utilizing BPVC VIII-2-2017. When determining stress ratios for the purpose of entering a table or figure in BPVC VIII-2-2017, ASME allowable stresses per ASME BPVC.II.D.C-2017 for Class 1 Vessels (no fatigue analysis) are applicable.

Annex R:

Revise Table R-1 as follows:

R.2.2.1.1 Materials for the following metallic components (including their penetrations, piping, anchors, stiffeners,and attachments) shall be selected from Table R-1 and shall be impact tested in accordance with R.2.2.2 through R.2.2.5, unless plates have impact test exemption per paragraph R.2.2.2.6.

R.2.2.2.1 Impact testing of plates including structural members made of plate, unless exempt from impact testing per R.2.2.2.6, shall comply with Table R-2.

Add R.2.2.2.6:

R.2.2.2.6 The Low-stress /Low-Temperature Provisions of 4.2.2 may be utilized.

R.2.3.2 Material for warm product vapor containers shall conform to one of the following:

a) Table 4-1 for design metal temperatures down to –35 °F (lowest 1-day mean ambient temperature of –35 °F)without impact test unless they are required by Table 4-1 or by the Purchaser.

b) Table R-3 for design metal temperatures down to –60 °F without impact tests unless they are required by Table R-4 or by the Purchaser.

c) May be selected and designed utilizing the Low-Stress /Low Temperature Provisions of 4.2.2.

Annex Q:

Q.2.2.1 Materials for the following metallic components (including their penetrations, piping, anchors, stiffeners, andattachments) shall be selected from Table Q-1 and shall comply with the requirements of Q.2.3, except that for a), b), c) when not lap-welded, and e) below, the Low-Stress /Low-Temperature Provisions of 4.2.2 may be alternatively utilized:

a) primary liquid containers;

b) secondary liquid containers;

c) refrigerated temperature roofs (this includes inner roofs of double roof tanks, and single roofs of tanks withexternal roof insulation);

e) for full containment tank systems; portions of warm product vapor containers that may experience cold gas flowsin the event of primary liquid container leakage;

f) metallic suspended decks for insulation;

Q.2.4.2 Material for warm product vapor containers shall conform to one of the following.

a) Table 4-1 for design metal temperatures down to –35 °F (lowest one-day mean ambient temperature of –35 °F)without impact tests unless they are required by Table 4-1 or by the Purchaser.

b) Table R-3 for design metal temperatures down to –60 °F without impact tests unless they are required by TableR-4 or by the Purchaser.

c) Paragraph Q.2.2 without impact tests unless they are specified by the Purchaser.

d) May be selected and designed utilizing the Low-stress /Low Temperature provisions of paragraph 4.2.2.

. . . End of API 620 Proposed Modifications

Examples:

1. Location: Alaska

Unheated Gasoline Tank. Shell Plate - SA 516-60 (not normalized), no impacts, no corrosion allowance.Low One Day Mean = -57 ºF (Note, by the way, this temperature is realistic)Look at Rings #4, 5, and 6 out of 7.Calculated Hoop Stress in the thickest ring, 0.75” thick Shell Ring #4 = 6 ksiDMT = LODMT + 15 ºF = -42 ºF

Questions: Is the shell plate ring #2 material SA 516-60 as presented acceptable?

a. Note: If impacts had been taken and they met the provisions of 4.2.5.1 second sentence and the above added part to Table 4-1, the low-stress /low-temperature provisions of 4.2.2 (b) would not need to be utilized.

b. Determine the MDMT for this material without impact tests and without PWHT using BPVC VIII-2 -17 as outlined above.Referring to BPVC VIII-2-17 Figure 3.7, the MDMT with no impacts = +10 ºF, so this material is not close to being OK at ‘full’ allowable stress.

c. So, see if the material may be utilized as provided using the low-stress /low-temperature provisions of 4.2.2 (b) referring to BPVC VIII-2-17.Normal Allowable Stress = 21.3 ksi (ASME II, Part D, Table 5A)Stress Ratio = 6 ksi /21.3 ksi = 0.28From Figure 3.12, a reduction of 52 ºF is allowedThus, MDMT for this stress condition may be reduced to +10 ºF - 52ºF = -42 ºF, So OK this is marginally acceptable.

d. Consider that the stress is 5 ksi instead of 6 ksi. Stress Ratio = 5 ksi/ 21.3 ksi = 0.235 < 0.24 from 3.11.2.5 Step 5) (a),so, the MDMT for this stress can be -155 ºF (-104 ºC).

2. Siberia Russia.

Unheated Gasoline TankLowest one day mean temperature -67 ºF. DMT = -67 ºF + 15 ºF = -52 ºF. This temperature is realistic, by the way, at this location. Refer to API 620 4.2.2 b). Consider Shell SA 516-70N Thickness = 1.0”, not impact tested. There is zero corrosion allowance. Tank is 6 rings high, with only lightly loaded attachments with attaching fillet welds <= 3/8” and NDE per 3.11.2.9 (b) (1). Shell vert and girth butt seams 100% RT. No PWHT planned.

a. What allowable stress may be used for the vertical butt welds for Rings 2 through 5, without impact testing?

Answer.Refer ASME VIII Div 2 Figure 3.7 Curve D Lowest MDMT with no impacts = -7 ºFThe DMT is well above below this exemption temperature of -7 ºF, so impact testing is required unless the stress is limited to 24% of the allowable stress of 25.3 ksi (from ASME II, Part D, Table 5A) = 6.1 ksi. PWHT: refer to 3.11.2.9 which allows certain exemptions to PWHT.

b. Assume Impact testing IS performed at -40 ºF. What allowable stress may be used for rings 2 through 5?

Answer.

We need to ‘accommodate’ another 12 ºF reduction to get from -40 to -52 ºF. Referring to BPVC VIII-2-2017 paragraph 3.11.2.5 and Figure 3.12, we can determine the stress reduction ratio = 0.85 relates to 12 ºF. So, allowable stress from ASME would be 0.85 * 25.3 = 21.5 ksi, but the allowable stress from API 620 governs and is 21 ksi.

c. For item B. 2 just above, what are the impact energy values required?

Refer to BPVC VIII-2-2017 Figure 3.3, showing the minimum energy level requirement of 20 ft-lbs (average of 3 specimens).

d. For Ring 1 which is attached to an annular plate, would it be allowed to have a hoop stress > 24% of the normal allowable stress without utilizing PWHT?

Answer: No, not allowed per BPVC VIII-2-2017 3.11.2.9 (a) unless PWHT is provided. And note that the corner weld would be Category C joint, which is not an ‘exempt’ type per 3.11.2.0 (b) (1).